1. Field of the Invention
The present invention relates to an EL (electroluminescence) display device obtained by fabricating semiconductor elements (elements formed by using a thin semiconductor film) on a substrate and to an electronic device having the EL display device as a display unit.
2. Prior Art
Technology has been greatly advanced in recent years for forming TFTs on a substrate, and attempts have been made to develop an active matrix-type display device. In particular, TFTs using a polysilicon film exhibit a higher electric-field mobility (also called mobility) than that of the conventional TFTs using an amorphous silicon film, and make it possible to accomplish a high-speed operation. This makes it possible to control the pixels, which has heretofore been done by a drive circuit outside the substrate, by using a drive circuit formed on the same substrate as the pixels.
The active matrix-type display device is drawing attention owing it its advantages such as a decrease in the cost of production, a decrease in the size of the display device, an increase in the yield and a decrease in the throughput, as a result of fabricating various circuits and elements on the same substrate.
The active matrix-type EL display devices have heretofore been employing pixels of a structure that is generally as shown in FIG. 3. In FIG. 3, reference numeral 301 denotes a TFT (hereinafter referred to as switching TFT) that works as a switching element, 302 denotes a TFT (hereinafter referred to as current control TFT) working as an element (current control element) for controlling the current supplied to an EL element 303, and 304 denotes a capacitor (holding capacity). The switching TFT 301 is connected to a gate wiring 305 and to a source wiring (data wiring) 306. The drain of the current control TFT 302 is connected to the EL element 303, and the source thereof is connected to a current feeder line 307.
When the gate wiring 305 is selected, the gate of the switching TFT 301 is opened, a data signal of the source wiring 306 is accumulated in the capacitor 304, and the gate of the current control TFT 302 is opened. After the gate of the switching TFT 301 is closed, the gate of the current control TFT 302 remains opened due to the electric charge accumulated in the capacitor 304 and, during this period, the EL element 303 emits light. The amount of light emitted by the EL element 303 varies depending on the amount of electric current that flows.
Here, the amount of current supplied to the EL element 303 is controlled by a gate voltage of the current control TFT 302 as shown in FIG. 4.
FIG. 4(A) is a graph illustrating transistor characteristics of the current control TFT, wherein a curve 401 represents Id-Vg characteristics (or an Id-Vg curve), Id represents a drain current and Vg represents a gate voltage. From this graph, it is possible to know the amount of current that flows relative to any gate voltage.
Usually, the EL element is driven by utilizing the Id-Vg characteristics over a region surrounded by a dotted line 402. FIG. 4(B) is a view illustrating the region surrounded by 402 on an enlarged scale.
In FIG. 4(B), the hatched region is called sub-threshold region. In practice, this region has a gate voltage close to or lower than the threshold voltage (Vth) and where the drain current exponentially varies depending upon a change in the gate voltage. In this region, the current is controlled based on the gate voltage.
The data signal input to the pixel as the switching TFT 301 is opened is, first, accumulated in the capacitor 304 and directly serves as a gate voltage for the current control TFT 302. Here, the drain current is determined for the gate voltage at a ratio of 1 to 1 in compliance with the Id-Vg characteristics shown in FIG. 4(A). That is, a predetermined current flows through the EL element 303 depending on the data signal, and the EL element 303 emits light in an amount corresponding to the amount of current.
Thus, the amount of light emitted by the EL element is controlled by the data signal, and the gradation display is accomplished by controlling the amount of light that is emitted. This system is a so-called analog gradation; i.e., the gradation display is accomplished relying on a change in the amplitude of the signal.
However, the analog gradation system has a defect that it is very susceptible to dispersion in the characteristics of the TFTs. For example, considered below is a case where a switching TFT that would exhibit the same gradation has ID-Vg characteristics different from those of a switching TFT of the neighboring pixel (generally shifted toward the positive side or the negative side).
In this case, the drain currents flowing into the switching TFTs vary depending upon the degree of dispersion, and different gate voltages are applied to the current control TFTs of the pixels. That is different currents flow into the EL elements and, as a result, light is emitted in different amounts making it no longer possible to accomplish the same gradation display.
Further, even when the same gate voltage is applied to the current control TFTs of the pixels, the same drain current cannot be produced if there is a dispersion in the Id-Vg characteristics of the current control TFTs. As will be obvious from FIG. 4(A), further, since use is made of the region where the drain current exponentially changes relative to the change in the gate voltage, even a slight difference in the Id-Vg characteristics results in a large change in the amount of current despite an equal gate voltage is applied. Then, the amount of light emitted by the EL elements greatly varies among the neighboring pixels.
In practice, the situation becomes more severe due to synergistic effect of dispersion of both the switching TFTs and the current control TFTs. Thus, the analog gradation system is very susceptible to the dispersion in the characteristics of the TFTs, hindering the attempt for realizing the multi-color active matrix EL display device.